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How Much Snow?

I was watching the news and a video of a roof collapsing caught my attention. There is some amazing physics in that one video. How do engineers determine how much snow a roof can hold and how do they make sure buildings are safe in most conditions?

I would begin this discussion with a rich resource from NOVA and PBS on why the towers fell. This would give some background on engineering, how buildings are constructed and how building codes fit into the mix.http://www.pbs.org/wgbh/nova/wtc/

To begin the search for snow loads there are several sites that are actually vendors of roof rakes (ask anyone from Minnesota). This site does a pretty good job of explaining roof loads and the variables (pitch age, construction).http://www.minnsnowta.com/snowloading.html

Now you are into the messy mathematics part of the question. How much does snow weigh? A one inch layer of water or ice weighs approximately 5 lb per square foot. Most sites tell us that a simple roof designed for a 20 lb per square foot snow load could in theory hold up to 4 inches of ice. Most snow has a moisture content somewhere around 12 inches of snow equaling one inch of water. Now the fun part is getting the students to measure a roof (or they can get the measurements of any of the school’s roofs by calling the maintenance guru for the school district.

This is where lots of questions begin. How much moisture was in the snow that fell? What is the snow load design of our school roof? How much snow would we have to have for the roof to need snow removal? The question on moisture content lives in earth science. Snow – water ratios can vary from 9:1 all the way to 14:1. The later being dry powdery snow. It would be great to take the students outside and have them measure the water content of the snow. There is actually a good site from the National Parks Service that gives an excellent activity on this topic.http://www.nps.gov/glac/forteachers/loader.cfm?csModule=security/getfile&pageid=425469

Students will have questions about their own houses. Flat roofs require no complex math but a pitched roof requires some scientific thinking. If a roof is pitched there is a possibility that the snow will slide off before it becomes too heavy. However, there are some interesting variables. My students mentioned the air temperature, the roof temperature, whether the house was well insulated, and wind. A house that is not well insulated will leak some heat to the roof and possibly melt the snow allowing it to slide off the pitched roof. This is a great time to have kids design a fair test. Using some shingles donated from the local roofing company and some lumber from the wood shop, you are ready to go outside and test a pitched roof. Add some heat under that wood and you can see if insulation impacts the snow melting.

We often fail to show kids how science works to keep them safe and how useful basic science knowledge is when applied. Likewise, we teach the sciences as separate disciplines. Seldom does science come packaged that way in problems. To solve any challenge scientists usually need to apply content from several disciplines. Maybe teaching it that way will help.